The present invention discloses a method and apparatus for modifying a video signal in a manner such that the modified video may be displayed in a normal fashion on a standard monitor device (television, video monitor, etc.), yet copies made of the modified video signal by videotape recorders will reproduce a video signal containing disturbances which cause a generally unviewable display on a standard television or other video monitor device.
The proliferation of the consumer video cassette recorder (VCR) within recent decades has contributed to a reduction in the value of broadcast television programming. The general public now has the facility with which to record programs broadcast via conventional terrestrial television, cable and satellite, and watch them at any time it is convenient. The consumer can retain a copy of a program and watch it multiple times, detracting from a broadcaster's customer base. The consumer can also eliminate commercial advertisements inserted into the program material, which is the primary basis upon which the conventional television industry has been supported. The consumer may also make additional copies of program material to provide to other consumers.
More recently, burgeoning television industries such as Pay Per View, Compressed Multi-Program, High Definition, Videodisc and Interactive Television have emerged. The expanding interest in the dissemination of video information has intensified the demand for a method of protecting this valuable copyrighted program material.
There have been many attempts to satisfy this requirement. There are an abundance of patents aimed at protecting video program material which has been broadcast via encryption/decryption schemes. Such designs require the addition of special decoding circuitry at the consumer's location, and thus are irrelevant to this discussion.
Numerous patents have been awarded which are aimed at protecting program material which has been prerecorded onto the distribution media of videotape and laser videodisc, as well as for designs aimed at protecting broadcast video program material. Most of these patents and their disadvantages have been referenced in the applicant's U.S. Pat. No. 5,251,041, issued Oct. 5, 1993. Patents which pertain more specifically to the present invention will be referenced at this time.
Processes which can generally be referred to as "time-base modulation" are exemplified in the U.S. Pat. Nos. 4,488,176, 4,673,981; 4,914,694 and 5,034,981, and the PCT application No. WO 90/04305. Certain portions of a video waveform are used by monitoring devices and VCR's to synchronize various circuits necessary for their proper operation. More specifically, the field rate which is derived from the vertical sync signal, and the line rate which is derived from the horizontal sync signal. In a VCR, these signals are used as references to adjust servo circuits which maintain correct tape and headwheel speed, and to correct for signal phase variations. Because these VCR circuits are generally more sensitive to sync timing variations than circuits used to synchronize scanning in a monitor device, several systems have been devised that exploit these differences. It is claimed that a video waveform modified by these systems will usually produce an acceptable display on a monitor device, while a copy of a thusly modified video signal recorded by a VCR, when played back, will exhibit speed and tracking fluctuations that produce a generally unacceptable display on a monitor device. While it is true that an unauthorized videotape recording of these signals will exhibit annoying disturbances in the viewed picture, other disturbing artifacts are produced by the copy protection process itself, which become evident while attempting to view the modified video signals, even before attempts are made to copy these signals.
Generally, these processes function to disturb a VCR's servo circuitry by modulating, or moving the position of the horizontal and/or vertical sync signals about their normal position. This is analogous to changing the frequency of the vertical and horizontal sync. The rationale is that a monitor device tends to be more forgiving of these deviations than a VCR. The standard frequency of these sync signals are the mean about which these frequencies are varied, both to reduce memory requirements within the digital conversion circuitry utilized, and to ensure that the program material does not vary too drastically from its normal elapsed time.
One problematic area with this approach is that by changing the sync frequency, the size of the picture is necessarily distorted. Compensation for geometric distortion caused by the fluctuating picture height, length and aspect ratio must be attempted, usually by dropping or adding pixels to picture elements. This involves highly complex circuitry required to determine the appropriateness of the compensation. To date, these types of logic designs have not achieved satisfactory performance, sometimes limiting the implementation of the time base modulation techniques to moments when there is a change in the picture scene, wherein it is hoped that the disturbances will be less noticeable.
Another area that has proven problematic is due to the fact that many monitor devices incorporate what is commonly referred to as a digital vertical countdown circuit. These circuits customarily compare successive vertical sync fields to determine what the vertical frequency is, before actually switching to a new vertical frequency. This is done primarily to improve the noise immunity of the vertical drive oscillator. Because there are many different vertical countdown arrangements, and because these designs undergo evolutionary changes, a wide variety of effects have been observed on different monitors when varying the vertical frequency. These anomalies can be caused by the detection of the standard vertical frequency when the vertical rate is modulated through this frequency, causing the monitor to attempt to display at a standard vertical rate, when in fact the video contains a changing, non-standard vertical rate. Other anomalies can be caused by the transition of changing the direction of the modulation of the vertical frequency, which must occur at some point in this scheme. If the shift from greater to less (or vice versa) vertical frequency is too abrupt, then monitors which incorporate a conventional analog-style vertical oscillator will not be able to react to the change quickly enough, producing violent "bouncing" in the displayed picture as the monitor attempts to adjust to the change. This can also occur in monitors incorporating digital vertical countdown circuits which revert to an analog mode when they cannot establish a fixed vertical rate.
Changing the number of lines contained within successive vertical fields can confound circuits utilized for decoding information sometimes placed within the vertical blanking interval. Such information includes closed captioning, videotext data, vertical interval reference signals (VIR), vertical interval test signals (VITS), vertical interval time code (VITC), station identification signals (SIDS), anti-ghost signals, etc. Decoding circuitry for these signals typically depend upon a standard vertical blanking interval for a reference with which to trigger a line counter. When the proper number of lines has elapsed, the decoder attempts to retrieve data from the line in which the data to be decoded is contained. Devices such as closed caption decoders will key text material over the active picture information so that the textual information is visible. A standard line count is assumed by these devices for accurate decoding and placement of decoded information. A non-standard and varying horizontal line count will cause unpredictable results.
Changing the horizontal frequency will necessarily change the horizontal line length. Many newer monitor devices contain a comb filter to provide for separation of the chroma and luminance signals, and for enhanced display of detail. Briefly, these comb filter circuits operate by comparing a given horizontal line to a previous horizontal line. The method of storing a previous horizontal line for comparison is usually accomplished by means of a glass delay line or charged coupled device. These storage devices have a fixed storage, or delay time equal to the length of a standard horizontal line. If the horizontal line length is modulated, as in some existing copy protection systems, artifacts will be produced in the displayed picture by the comb filter due to non-correlation between the varying length horizontal lines.
Changing the vertical frequency of a video signal will necessarily change the height of a displayed picture, causing a corresponding change in the duty cycle of the vertical deflection current. This can produce a shift in the quiescent center of the displayed picture and thus destroy the accuracy of the interlaced scanning system. Changing the size of a displayed picture also requires that the active video elements be shifted up or down accordingly, so that the positional movement of successive picture elements will be minimized. Because of differing monitor designs and their resultant reactions to these changes, it is difficult to envision a system of this type that will perform with similar ballistics on all existing monitor devices.
The above mentioned disadvantages detract from the practicality and desirability of these currently available copy protection systems.